Regional Trends of Biological Resources  Grasslands

Prairie Grasses

Tall-grass Prairie

Fig. 3. Tall-grass prairie.

Tall-grass prairie (Fig. 3) is the wettest of the grassland provinces and
is predominantly composed of sod-forming bunch grasses. Like other grasslands,
the tall-grass prairie has species originally from different geographical
sources (Simms 1988). Grassland groupings of the tall-grass prairie are the
bluestem prairie from southern Manitoba through eastern North Dakota and western
Minnesota south to eastern Oklahoma, and the wheatgrass, bluestem, and needlegrass
area from south-central Canada through east-central North Dakota and South
Dakota to southern Nebraska.

Three additional areas are associated with tall-grass prairie: the Crosstimbers,
a band of grassland and oak savanna at the southern edge of the bluestem prairie
in Kansas to the Trinity River in Texas (KŘehler 1964), the Blackland Prairie
south of the Crosstimbers (Gould 1962), and the rice prairies. The rice prairies
are former coastal prairies that have been converted to rice production (Hobaugh
et al. 1989). The original vegetation in rice prairies was mainly tall grass
and extended across 9,000 square kilometers, largely along the Texas coast
and inland as much as 125 kilometers, and into Louisiana. Little coastal prairie
remains; Attwater's Prairie Chicken National Wildlife Refuge in Texas is the
single major remnant.

Since 1830 declines in the area of tall-grass prairie within specific states
and provinces are estimated to be 82.6 to 99.9% (Table 1) and exceed those
reported for any other major ecological community in North America (Samson
and Knopf 1994). Iowa, for example, has barely 12,140 hectares remaining of
its original 12 million hectares of tall-grass prairie. Less than 1% of the
presettlement tall-grass prairie remains in Manitoba, Illinois, Indiana, and
North Dakota. Minnesota and Missouri, states active in prairie conservation,
work with less than 9% of the presettlement tall-grass prairie. Tall-grass
prairie remains important to ranching in the Osage and Flint Hills of Kansas
and in tracts in South Dakota, Oklahoma, and Texas.

Mixed-grass Prairie

One can envision the short-grass and tall-grass
prairies intergrading just east of an irregular line that runs from
northern Texas through Oklahoma, Kansas, and Nebraska, northwestward
into west-central North Dakota and South Dakota (Figs. 1
and 2). The perimeter is not well defined
because of the array of short-stature, intermediate, and tall-grass
species that make up an ecotone between the short-grass and tall-grass
prairies (Bragg and Steuter 1996). In general, the mixed-grass prairie
is characterized by the warm-season grasses of the short-grass prairie
to the west and the cool- and warm-season grasses, which grow much taller,
to the east (Fig. 4). Because of this ecotonal mixing, the number of
plant species found in mixed-grass prairies exceeds that in other prairie
types. Estimated declines in area of native mixed-grass prairie, although
less than those of the tall-grass, range from 30.5% in Texas to over
99.9% in Manitoba (Table 1).

Fig. 4. Mixed-grass prairie in
Nebraska Sandhills.

Short-grass Prairie

Fig. 5. Short-grass prairie in Laramie
Plain, Wyoming.

The short-grass prairie extends east from the Rocky Mountains and south from
Montana through the Nebraska panhandle and southeastern Wyoming into the high
plains of Oklahoma, New Mexico, and Texas (Figs. 1
and 2.). The short-grass prairie landscape
(Fig. 5) was one of relatively treeless stream bottoms and uplands dominated
by blue grama and buffalo grass, two warm-season grasses that flourish under
intensive grazing (Weaver et al. 1996). Buffalo grass reproduces both sexually
and by tillering sprouts from the base of grass clumps. Unlike the more eastern
species, short-grass prairie species remain digestible and retain their protein
content when dormant.

Declines in short-grass prairie have generally been much less than those
of tall-grass and mixed-grass prairies (Table 1). However, perhaps in no other
system than short-grass prairie are historical and evolutionary impacts of
grazing so apparent (Knopf 1996). Clearly, birds endemic to the short-grass
prairie express life-history characteristics and habitat use in response to
grazing (Fig. 6). The mountain plover responds to highly disturbed sites,
the chestnut-collared longspur to moderately grazed areas, and the Baird's
sparrow to sites with taller grasses. In the mid-1800's the numbers of individuals
of native mammal speciesbison, prairie dogs, pronghorn, elk, grizzly
bears, and gray wolvesrivaled or exceeded those now in the African Serengeti
(Howe 1994). Major antigrazing structures evolved in plants: thorns and spikes;
thick or hard tissues difficult to bite, chew, or digest; and secondary compounds
difficult to digest. These structures have arisen through the long coevolutionary
association between plants and animals with grazing on grasslands.

At present, extensive areas of short-grass prairie are dominated by invasive
perennial and annual species, whose presence is attributed to overgrazing
by domestic livestock and dryland farming (Weaver et al. 1996). To the south,
specifically the Texas high plains, much of the short-grass prairie is now
farmland or shrubland invaded by prickly pear cacti and oaks. Only the short-grass
prairie and, to a lesser extent, mixed-grass prairie remain in public ownership.
These areas are largely on the national grasslands managed by the U.S. Forest
Service.